This invention relates to an exhaust gas measurement system, and more particularly, the invention relates to a system for more accurately measuring hydrocarbon emissions in vehicle exhaust.
Modern vehicles must meet stringent government guidelines that set forth the allowable amount of products of combustion in the vehicle's exhaust, such as CO, NOx, and hydrocarbons. Not only does the government set the permissible amount of each product of combustion, but the government also mandates some of the testing equipment and procedures that must be employed when measuring vehicle exhaust to ensure that the vehicle complies with the government rules. For example, the EPA has promulgated rules governing emissions from heavy-duty diesel-fueled engines as set forth at 40 CFR §§86.1310-2007. While the Rules permit measurement systems of various configurations, the Rules also mandate that particular procedures must be used.
The challenge in ensuring compliance with emissions guidelines is accurately measuring the products of combustion during transient vehicle operating conditions. That is, the products of combustion change as the vehicle is run through a federally mandated test procedure. This challenge is further complicated by the EPA's desire to lower the permissible amount of products of combustion and the limitations of current testing equipment. Many in the automobile industry object to the new Rules as being very difficult to comply with and fear that inaccuracies would result causing complying vehicles to fail. Current configurations of emissions testing equipment cannot meet the EPA's proposed requirements regarding hydrocarbon measurement, and new systems must be developed.
A typical constant volume emissions sampler is shown in Figure N07-1 in the Rules referred to above. The sampler includes a probe that collects exhaust gases from a vehicle exhaust pipe. The Rules describe one system for continuous hydrocarbon measurement used to measure the entire vehicle exhaust volume in which the exhaust gases flow through conduits to an inline heated flame ionization detector (HFID) to measure the hydrocarbons. The conduits and other system component leading up to and including the HFID typically must be heated to 191° C. to prevent the larger chain hydrocarbons from condensing within the system prior to measurement. The analyzer signal is integrated over the duration of the test to determine the quantity of hydrocarbons. Venturis are typically used to control the flow rate of the gases through the measuring system, which assumes a constant flow rate, although transients during the test prevent a constant flow rate from being maintained. As a result, inaccuracies in continuously measuring the products of combustion in line with the HFID occur, which may cause some vehicles to fail the test when they should have passed and vice versa because of the dynamic nature of the emissions the HC analyzer has to be operated in a relatively high range.
The Rules describe another system for measuring hydrocarbons that employ plastic bags that store only a portion of the vehicle's exhaust. The exhaust gases are collected in the bags and then after the completion of the test the bags are transferred through conduits to an analyzer where all of the hydrocarbons may be measured. Collection of the gases within the bags is a “pneumatic integration” of the diluted engine exhaust; the concentration in the bags is constant, which makes the use of a low analyzer range possible thereby increasing the accuracy. Again, much of the exhaust measurement system is heated to 191° C. to prevent the gasses from condensing. The Rules require that the bags also must be heated to 191° C. However, bags currently available may only be heated to about 40° C., which would only permit the smaller chain hydrocarbons to be measured. Hydrocarbons will be absorbed in the bag material, which reduces accuracy.
Using stainless steel canisters for storage of hydrocarbons at up to 160° for subsequent measurement has been proposed in the prior art. The canister is vacated and filled at a constant flow rate. When the canister is above atmospheric pressure, the exhaust gas is vented from the canister to an analyzer to measure the. Since the canister is a rigid container, it is not operated at constant atmospheric pressure as is a typical plastic bag, which makes it difficult to accurately control the flow rate and measure the sample. The disclosed system does not meet the requirements of the Rules, nor does the system disclose how the system accurately controls the flow rate of the exhaust gases into and out of the canister. Therefore, what is needed is an exhaust gas measurement system that meets the requirements of the Rules while providing accurate results to the automobile industry.